The assembled MJ: Files id to stamp, or null to leave the session's file unchanged.
The gaps to store. An array is serialized to JSON; a string (already-serialized JSON) or null is
stored verbatim, which is how a Pending hand-back keeps the gaps of the recording already stored.
The version IRealtimeRecordingLedger.ClaimAssembly returned; the finish is fenced on it.
Input to IRealtimeRecordingLedger.FinishAssembly.